Diamonds are often graded according to four “C's,” which correspond to the diamond's color, clarity, carat weight, and cut. Color generally refers to the diamonds hue, and is related to impurities in the diamond's composition. Clarity generally refers to defects, such as bulk or surface defects, which can scatter light and reduce a diamond's visual appeal. The cut generally refers to both the diamond's shape (e.g., round, marquise, oval, etc.), and also to the diamond's proportions, symmetry, and polish. These parameters influence how light interacts with the diamond, which facets appear bright to an observer, which facets appear dark, and which facets appear colored.